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Old 02-16-2008, 05:12 PM   #1
zacks62fairlane
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Default sohc vs dohc

i was wondering what the power difference between the two is. i was also wondering what it takes to swap the two different heads? just direct swap?
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Old 02-16-2008, 07:32 PM   #2
FuJi K
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'96 2.5L NA DOHC head VS '99-'07 2.5L NA SOHC head
the DOHC head will outflow the SOHC head up top The 2.5L DOHC manifold is BIGGER than the SOHC manifold. The '97-'99 2.5L NA DOHC head is slightly smaller but still has bigger port runners & manifold than the 2.5L SOHC head.

I think running the WRX heads on the 2.5L NA would benefit more. I ALWAYS wonderd what parts are swappable over from the SOHC manifold to the WRX maniold. The WRX manifold is BIGGER and the WRX HEAD can be ported to be BIGGER and you'll gain it there w/ BIGGER manifold and head ports. The exhaust ports are also BIGGER if not the same size as the 2.5L SOHC heads.

I'm still not sure if the NEW DOHC heads ('02-'05) cam sensor readings are the same as the phase 1 DOHC. We where able to swap a '96 DOHC 2.5L NA into a '93 Impreza L 1.8L SOHC. Everything ran and it just had to learn the fuel trim. We just used the 1.8L engine harness and 1.8L throttle body.
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Old 02-17-2008, 10:46 AM   #3
RaceFaceXC
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i thought the pahse II EJ25 outflowed the phase I on n/a setup. There is a thread on here somewhere with info charts about flow tests and the phase II and I think the phase I only outflowed the Phase II heads above 6k rpm. ill see if i can find it. Im assuming your interested in weather your 07's SOHC will gain anything from going to a DOHC head?
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Old 02-17-2008, 10:47 AM   #4
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EJ25
Phase I - DOHC
Phase II - SOHC
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Old 02-17-2008, 07:21 PM   #5
FuJi K
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Quote:
Originally Posted by RaceFaceXC View Post
i thought the pahse II EJ25 outflowed the phase I on n/a setup. There is a thread on here somewhere with info charts about flow tests and the phase II and I think the phase I only outflowed the Phase II heads above 6k rpm.
They flow VERY close. The phase 2 SOHC has better exhaust ports than the PHASE 1 DOHC heads.
The SOHC has better velocity of getting air in when at lower revs. The bigger intake ports of the DOHC heads allows it to get air in at higher revs but the exhaust side kind of chokes it. I have a '01 2.5L SOHC head sitting in the basement and from visually looking at the exhaust ports, it IS less of a restriction compared to the '98 2.5L DOHC heads I had.

Here's the CFM chart you where looking for
http://forums.nasioc.com/forums/show...5&postcount=15
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Old 02-17-2008, 07:23 PM   #6
zacks62fairlane
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Quote:
Originally Posted by FuJi K View Post
They flow VERY close. The phase 2 SOHC has better exhaust ports than the PHASE 1 DOHC heads.
The SOHC has better velocity of getting air in when at lower revs. The bigger intake ports of the DOHC heads allows it to get air in at higher revs but the exhaust side kind of chokes it. I have a '01 2.5L SOHC head sitting in the basement and from visually looking at the exhaust ports, it IS less of a restriction compared to the '98 2.5L DOHC heads I had.

Here's the CFM chart you where looking for
http://forums.nasioc.com/forums/show...5&postcount=15
this makes sense as to why they use the dohc for the turboed motors... thanks for the info man
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Old 02-17-2008, 09:44 PM   #7
FuJi K
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Quote:
Originally Posted by zacks62fairlane View Post
this makes sense as to why they use the dohc for the turboed motors... thanks for the info man
BUT the PHASE 2 DOHC heads (ex: '02+ WRX) have the bigger exhaust ports as well. The restriction is where the valve guide is. There's LESS material around the valve guide on the PHASE 2 heads. The PHASE 1 2.5L DOHC head has more valve guide support/material, which restricts airflow at higher revs.

If you look here... you'll see the material around the valve guide of a '98 2.5L DOHC NA head.


PHASE1 JDM turbo head


If you look here...my stock '01 2.5RS heads (sorry for the carbon BTW).


Same for a USDM WRX head in this pic from here

Last edited by FuJi K; 02-25-2008 at 09:03 PM.
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Old 02-25-2008, 09:03 PM   #8
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^^^^ little update on the pics
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Old 02-25-2008, 09:46 PM   #9
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seems like the merged porting would benefit quite a bit from a nice P/P job. especially on the n/a engines. am i correct?
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Old 02-25-2008, 10:37 PM   #10
FuJi K
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Merged porting??? sorry I'm slow. The exhaust would benefit from a POLISHING job. The PHASE 1 heads seems to have more valve guide support than the PHASE2 heads, which somewhat hinders flow.
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Old 02-25-2008, 10:42 PM   #11
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Someone stole my WRX
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